The present invention relates to a color image forming apparatus wherein a toner image is formed on an image forming member through an electrophotographic system and the toner image is transferred onto a transfer material for obtaining a color image.
Owing to advancement of technologies, color image forming apparatuses have been developed by manufacturers recently. For example, Japanese Patent Publication Open to Public Inspection Nos. 100770/1985, 149972/1986 and 75850/1985 (hereinafter referred to as Japanese Patent O.P.I. Publication) disclose methods for obtaining color copies from a color document, wherein based on image data obtained by reading a document by means of scanning by a scanner of an image reading unit, latent image formation and development thereof are conducted on a photoreceptor drum for the frequency corresponding to the number of colors separated and transferred onto a transfer material to obtain a color image by superposing toner images each having its own color.
Incidentally, in an office where a image forming apparatus is usually used, popular paper sizes each forming an image of one unit include ordinary paper sizes (JIS standards) of A-3, A-4, B-4 and B-5. Therefore, the circumferential length and the width of an image forming member in an image forming apparatus such as a copying machine or the like need to be in size covering at least one of the aforementioned paper sizes. Namely, the circumferential length of the image forming member must include, in addition to the length of the maximum paper size, additional length such as the one for the standard toner image forming area for toner concentration detection and the one corresponding to a period of time necessary for a scanner of an image reading unit to return. The additional length necessary for the scanner to return occupies the greatest part of the total additional length.
However, the printing out speed in an image forming apparatus is the same for all paper sizes. For example, the printing out speed for A-3 size paper is exactly the same as that for A-4 size paper although the length of A-4 size paper is 210 mm which is a half the size of A-3 size paper. The reason behind this is that only a portion of the image forming member is used for image forming. For example, when printing out A-4 size paper under the condition that the maximum paper size is A-3, more than half of the image forming member is not used.
To be concrete, the image forming speed in the image forming apparatus is always the same regardless of the size of an image to be formed on the image forming member, which has been wasteful. In other words, the circumferential length of the image forming member is determined to cope with the maximum image size to be formed, and when the image size is large, the entire surface of the image forming member is used for image forming, while when the image size is small, only a small portion of the image forming member is used for image forming, resulting in a wasteful portion of the image forming member.
Furthermore, if the recent trend of adding high functions to a color image forming apparatus is provided, in addition to its original copy mode, with a print mode as a printer function for forming an image on a transfer material through the image forming system of the image forming apparatus using image data from an output device such as a computer or a word processor, it is not necessary to consider the width for a scanner to return when forming an image using image data from a peripheral output device.
The present invention has been achieved to solve the aforementioned problems, and its object is to provide a color image forming apparatus forming an image of a smaller size through the process that is more complicated than the process for an image of a larger size, wherein the number of image frames formed in a single operation is increased, and thereby the wasteful portion on an image forming member is eliminated and efficiency of operation is enhanced. Another object of the invention is to provide a color image forming apparatus wherein the circumferential length of an image forming member is minimized, thus, the apparatus may be caused, to be small in size and to be light in weight. A further object of the invention is to provide a color image forming apparatus capable of forming images with an optimum frequency of image forming and with high efficiency, in all image forming modes.